1. Field of the Invention
This invention relates to the production of a tooth restorative material and more particularly to the production of synthetic hydroxyapatite.
2. Description of the Prior Art
Traditionally, tooth cavities have been filled with silver or gold amalgam materials. Such amalgams are, however, far from ideal tooth restorative materials. From a cosmetic point of view, for example, these materials do not match the existing teeth in color. From a functional point of view, they are also inferior since their physical properties, such as heat conductivity, coefficient of expansion, etc., are markedly different from the physical properties of tooth enamel.
Recently, researchers have attempted to develop new tooth restorative materials having better properties than the existing silver or gold amalgams. Three such materials include the silica cements, methyl methacrylate based resins, and combinations of fused silica powder bound together in cross-linked organic polymers. The latter are thoroughly described in patents issued to Rafael L. Bowen. See U.S. Pat. Nos. 3,066,112; 3,194,783; and, 3,194,784.
Despite these attempts to produce better tooth restorative materials, the compositions produced to date have not proven to be entirely satisfactory. This is probably because of the many stringent requirements which must be met by successful tooth restorative materials.
The best possible dental restorative would be synthetically produced tooth enamel. Natural tooth enamel is considered to be tightly packed crystals of hydroxyapatite. People have, of course, recognized this and attempted to grow crystalline hydroxyapatite synthetically. While such growth has been accomplished, the yields have been exceptionally small and the syntheses have required long lengths of time, high pressurs and/or high temperatures.
Several prior art attempts to prepare crystalline hydroxyapatite synthetically are described in the literature. Thus, it has been shown that the formation of crystalline hydroxyapatite can be divided into two distinct stages in aqueous solution from the dissolution of non-crystalline calcium phosphate. In the first stage, the crystals grow by a diffusion controlled dendritic mechanism resulting in the production of a colloidal apatite particle. These apatite crystals continue to grow through a consolidation process known as Ostwald ripening. This process is exceptionally slow, as noted by the statements by the authors that the crystals increased in size from 120 A at the onset of the post-conversion period to only 251 A by the end of 70 days. See Eanes, E. D. and Posner, A. S.; "A Note on the Crystal Growth of Hydroxyapatite Precipitated from Aqueous Solutions;" Materials Research Bulletin; Vol. 5; pp. 377-384; 1970; Pergamon Press.
In another prior art process, synthetic, crystalline hydroxyapatite is formed from calcium phosphates under conditions of high pH and high initial concentrations of reactants. Three stages are involved and the third stage, which involves the conversion of an amorphous precursor to crystalline hydroxyapatite, doesn't begin until after seven hours of reaction, and is said to continue indefinitely by the authors. See Eanes, E. D. Gillessen, I. H. and Posner, A. S.; "Intermediate States in the Precipitation of Hydroxyapatite;" Nature; Vol. 5008; Oct. 23, 1965; pp. 365-367.
The slow nature of these syntheses is further described by Nancollas, G. H. and Mohan, M. S. in "The Growth of Hydroxyapatite Crystals;" Archives of Oral Biology; Vol. 15; pp. 731-745; 1970; Pergamon Press.
Some attempts to use hydrothermal bombs have been described. These employ extremely large pressures and high temperatures, but still have resulted in low yields and long times. See, for example: Kirn, J. F. and Keidheiser, Jr., H.; "Progress in Efforts to Grow Large Single Crystals of Hydroxyapatite;" Journal of Crystal Growth; Vol. 2; pp. 111-112; 1968; North-Holland Publishing Co.; and, Perloff, A. and Posner, A. S.; "Preparation of Pure Hydroxyapatite Crystals;" pp. 583-584.
As can be appreciated from the above discussion, there is a great need for a simple, controllable, reliable and speedy synthesis for crystalline hydroxyapatite. In fact, the Perloff and Posner article cited above states: "A simple method for the production of pure well-crystallized hydroxyapatites has long been needed."